1. Field of the Invention
The present invention relates to Luneburg lenses and more particularly it concerns a method of forming a Luneburg lens having an adjusted focal length.
2. Description of the Prior Art
Integrated optics is an attractive approach for providing new devices for signal processing such as scanners, deflectors, modulators, switches, RF spectrum analyzers, convolvers, correlators, multiplexers and demultiplexers, because of the potential for very high performance operation and high speed processing using optical processing principles in a structure that can be very small and rigid in its planar configuration, and which can be manufactured by batch fabrication techniques.
Such devices require thin-film waveguide lenses to control the shape of the guided beam for imaging, spatial filtering, focusing and Fourier analysis. The lenses for these applications must have high efficiency, high performance and high stability. Further, accuracy, meaning that the focal length of the lens shape is accurate enough to satisfy design specifications, is essential for more precise applications such as where a well-collimated guided beam or a sufficiently small beam spot size are needed.
One typical class of integrated optical lenses often considered for such use is the Luneburg lens. The Luneburg lens is one of the classical index-graded lenses and has a circularly symmetric refractive index distribution that perfectly focuses the arc of a fixed circle onto the arc of a second fixed circle. Such Luneburg lenses are fabricated by sputtering or evaporating the lens material onto a waveguide surface through a circular mask with shaped edges. S. K. Yao, et al., Guided-Wave Optical Thin-Film Luneburg Lenses: Fabrication Technique and Properties, Appl. Optics, 18, 4067 (1979).
However, a fabricated Luneburg lens often has a different focal length from the design value because of, for example, misalignment between mask and substrate positions or slightly different refractive indices caused by accidental error of the deposition condition.
These problems are usually very difficult to correct in the fabrication process, even when extreme care is taken. Therefore there is a need to somehow adjust the focal length of a fabricated Luneburg lens to correct for any error in the fabricated system. Otherwise the overall cost of fabrication of such devices becomes extremely expensive.